gdb/xcoffread: replace 2 switches with if

Replace these 2 switches that have only one case with a single if
statement.  It is highly unlikely that we'll ever need to check for
other values.

Change-Id: I6a48d4f27ba4d1d128aaa39d5f3d693a11e99fc9
Approved-By: Tom Tromey <tom@tromey.com>